Black pigments are iron-(II,III) oxides having a magnetite structure. In addition, mixed oxides rich in iron also exist, such as bixbyite, a manganese-containing iron-(III) oxide, which likewise produce very bluish-tinged black pigments. Iron oxide black pigments are generally produced by oxidizing metallic and/or divalent iron (precipitation process or aniline process) or by reacting trivalent iron compounds, optionally under reduction conditions (calcination or thermolysis process).
The conventional production of iron-(II,III) oxides having a magnetite structure from an aqueous medium is by the oxidation of iron-(II) salts at elevated temperatures (Masao Kiyama, Bull. chem. Soc. Japan, Vol 47 (7), pages 1646 to 1605 (1974). In this process, concentrated iron salt solutions are generally used as starting materials. Such solutions occur, for example, as waste solutions in pickling plants or as titanium-containing by-product in the production of titanium dioxide by the so-called "sulphate process". The crude materials are correspondingly predominantly iron-(II) sulphates and/or chlorides generally having concentrations of from 150 to 250 g of salt/l in the case of iron-(II) sulphate and from 200 to 400 g of salt/l in the case of iron-(II) chloride.
In the present context, acidic waste waters are iron-containing waters having metal salt contents of less than 100 g/l, preferably less than 25 g/l. These occur, for example, in the production of iron oxide pigments from washing filtrates. Water from sewage works, which in a third stage has to be subjected to a subsequent precipitation with adsorption on iron hydroxide for clearing and purifying heavy metals and phosphate ions, is also a suitable starting material according to the present invention. A common feature of all the waste waters is that the amount of water, as compared to the amount of iron to be separated, is so large that an economical production of pigment quality iron oxide by the precipitation process with a temperature increase of the salt solution of greater than 50.degree. C. is not possible. Such waste waters are conventionally neutralized at ambient temperature under oxidation conditions (possibly recovering contact slurry), so as to dissociate all iron ions including foreign ions. Due to the inhomogeneity of such precipitates (all phases between iron oxide, iron oxide hydroxide and pure iron hydroxide may be present), they are not suitable as starting materials for ferrites and pigments.
An object of the present invention is thus to provide a process which allows the economical production of high-quality iron oxide pigments from the waste waters thereof.